Brandywine GPS8 Plus User manual

MANUAL P/N 900000007 REV 2.0
User Guide
GPS Time and Frequency System
Model GPS8 Plus
P/N 071000003
Revision 2.0
October 2004
Brandywine Communications
2230 South Fairview Street
Santa Ana, CA 92704
[phone]
[phone]
http://www.brandywinecomm.com

MANUAL P/N 900000007 REV 2.0
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Revision History
REVISION DATE COMMENTS
1.9 09-16-04 Preliminary release of GPS8 Plus user guide.
2.0 10-11-04 Revision of entire user guide.

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Safety Warnings
WARNING: This unit contains lethal AC voltages.
Disconnect the unit from the AC supply before removing
the cover.
WARNING:
The lightning flash with an arrowhead inside of an equilateral triangle is
intended to alert the user to the presence of un-insulated “dangerous
voltage” within the product’s enclosure. The “dangerous voltage” may be
of sufficient magnitude to constitute as a risk of electrical shock to people.
CAUTION:
The exclamation point inside of an equilateral triangle is intended to alert
the user to the presence of important operation and maintenance
instructions in the user guide.

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Table of Contents
1Introduction ............................................................................................................................. 6
1.1 Scope of Section ............................................................................................................ 6
1.2 Purpose of Equipment.................................................................................................... 6
1.3 Specification ...................................................................................................................7
1.4 Controls, Indicators, and Connectors........................................................................... 11
2Installation............................................................................................................................. 13
2.1 Scope of Section .......................................................................................................... 13
2.2 Unpacking and Inspection............................................................................................ 13
2.3 Installation and Testing ................................................................................................ 13
3Operating Instructions........................................................................................................... 14
3.1 Scope of Section .......................................................................................................... 14
3.2 Operation...................................................................................................................... 14
3.3 Serial Communications ................................................................................................ 17
3.4 TxD1/RxD1 (Rear Panel J13) Baud Rate and Character Format................................ 18
3.5 TxD1/RxD1 (Rear Panel J13) User Commands and GPS8 Plus Responses ............. 19
3.6 Message Formats......................................................................................................... 22
3.6.1 Time and Date with Status, Position Averager Status, and Output Status ......... 22
3.6.2 Sounder Control .................................................................................................. 23
3.6.3 1
st
Pulse Output Data and Command (J10) ........................................................ 23
3.6.4 Dilution of Precision Values and Satellites Used................................................. 26
3.6.5 Time Code and Serial Data Output Formats ....................................................... 27
3.6.6 Frequency and Phase Controller Data ................................................................ 30
3.6.7 Health Status of Satellites ................................................................................... 36
3.6.8 Leap Second Information (Firmware 13+)........................................................... 37
3.6.9 2
nd
Pulse Output Data and Command (PCB CON6 Pin 3).................................. 38
3.6.10 Location and Signal Noise Ratio of Satellites...................................................... 39
3.6.11 Firmware Version Number................................................................................... 40
3.6.12 Offset of Local Time Data and Command........................................................... 41
3.6.13 Position of GPS Antenna..................................................................................... 42
3.6.14 Additional Information (Including Magnetic Declination) ..................................... 43
3.6.15 Request Precision Time ...................................................................................... 43
3.6.16 Software Reset Command .................................................................................. 44
3.7 TxD2/RxD2 Messages (Rear Panel J12) ..................................................................... 45
3.7.1 Type 1 Format ..................................................................................................... 45
3.7.2 Type 2 Format ..................................................................................................... 45
3.7.3 Type 3 Format (GPS4 Format)............................................................................ 47
3.8 IRIG B and AFNOR NF 2 87-500 Time Code Output .................................................. 48
3.8.1 IRIG B and AFNOR NF 2 87-500 Time Code Bit Allocation ............................... 49
3.9 NASA36 Time Code Output ......................................................................................... 51
3.10 2137 Time Code Output............................................................................................... 53
3.11 VELA Time Code Output (Slow Code)......................................................................... 54
3.12 Alarm Contacts (J14) ................................................................................................... 55
4GPS8 Plus Keypad and Display Operation .......................................................................... 56
4.1 Scope of Section .......................................................................................................... 56
4.2 General Principles of Operation of Keypad and Display.............................................. 56
4.3 Messages after Power On............................................................................................ 57
4.3.1 Operating Mode Selection ................................................................................... 58
4.3.2 GPS Locked Mode .............................................................................................. 58
4.3.3 Free Run Mode.................................................................................................... 59
4.3.4 1 PPS Lock Mode................................................................................................ 59
4.3.5 Manual Time and Date Entry............................................................................... 59
4.4 Position Screen ............................................................................................................ 60
4.5 Satellites Tracked Screen ............................................................................................ 60
4.6 1 PPS Status Screen ................................................................................................... 61
4.7 Mean Phase Screen..................................................................................................... 62
4.8 Estimated Mean Frequency ......................................................................................... 63

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4.9 Frequency Control........................................................................................................ 63
4.9.1 Test Configuration of Frequency Controller ........................................................ 64
4.10 Position Mode Configuration ........................................................................................ 64
4.10.1 Position Mode Selection ...................................................................................... 65
4.11 Local Time Configuration ............................................................................................. 65
4.11.1 Local Time Offset Adjustment ............................................................................. 65
4.12 Time Code Configuration ............................................................................................. 66
4.12.1 Time Code Selection ........................................................................................... 66
4.12.2 J13 TXD1 Configuration ...................................................................................... 67
4.12.3 J12 TXD2 Configuration ...................................................................................... 67
4.13 J10 Pulse Output Configuration ................................................................................... 68
4.13.1 J10 Timed Pulse Configuration ........................................................................... 68
4.13.2 J10 Pulse Period Configuration........................................................................... 69
4.13.3 J10 Pulse Time Code Configuration.................................................................... 70
4.14 Status Screen and Alarm Configuration 1.................................................................... 71
4.14.1 Selection of Alarm Masks 1................................................................................. 71
4.15 Status Screen and Alarm Configuration 2.................................................................... 72
4.15.1 Selection of Alarm Masks 2................................................................................. 72
4.16 Status Screen 3............................................................................................................ 73
4.17 Status Screen 4............................................................................................................ 73
5Principles of Operation.......................................................................................................... 74
5.1 Scope of Section .......................................................................................................... 74
5.2 Theory of Operation ..................................................................................................... 74
6Maintenance and Calibration ................................................................................................ 77
6.1 Scope of Section .......................................................................................................... 77
6.2 Routine Maintenance ................................................................................................... 77
6.3 Fault Finding................................................................................................................. 78
6.4 Oscillator Module.......................................................................................................... 78
7Diagrams............................................................................................................................... 79
8Appendix I Output Alarm Masks ........................................................................................... 86
9Appendix II Phase Noise....................................................................................................... 90

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1 Introduction
1.1 Scope of Section
Section 1 provides a general description of the GPS8 Plus. The introduction is
divided into three parts; purpose of the equipment, physical and electrical
specification, and an identification of the external controls, indicators, and
connectors.
1.2 Purpose of Equipment
The GPS8 Plus is a multi-output precision time and frequency standard that uses
the Global Positioning System (GPS) to steer and hold an internal oscillator and
clock system precisely on time. It is designed for use in telecommunications,
power utility, and military communication applications. Time and frequency
information is maintained to high accuracy by the internal oscillator even if no
satellites can be tracked. Each space vehicle in the GPS system transmits time,
frequency, and navigation information in two formats.
•P code: a precision code restricted to authorized users.
•C/A code: an unrestricted code available for general use.
In both cases the time information is maintained by the USA Department of
Defense and is traceable to the USA Naval Observatory. The standard GPS8
Plus uses the C/A code.
The GPS8 Plus provides a wide range of output options to form a complete time
and frequency management system. In addition, satellite data and navigational
information is outputted in the form of longitude, latitude, and altitude for
applications where precise positions must be known. For fixed position
operation, precise averaging of the position is carried out by the GPS8 Plus over
24 hours.

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1.3 Specification
SPECIFICATION DESCRIPTION OPTION
Satellite Signal GPS L1 1.57542 GHz
Satellite Code C/A 1.023 MHz
Receiver Type Parallel eight channels (8 satellites tracked
continuously and simultaneously)
Receiver Sensitivity -133 dBm
Input Impedance 50 ohm
Standard Antenna Active omni-directional 30 dB gain
Cold Start Requirement Automatic (no time or position input needed)
Typically 8 minutes to time synchronization
Satellite Acquisition Time Typically < 20 seconds (warm start)
Data Update Rate 1 per second
Timing Accuracy Tracking
Satellites (XTAL_1)
Within ±150 ns of GPS time
Standard deviation of 34 ns
Rubidium (Rb_2) oscillator
Better than 200 ns per hour
High stability OCXO (XTAL_1) oscillator
Better than 1 µs per hour
Standard OCXO (XTAL_2) oscillator
Better than 10 µs per hour
Timing Stability Tracking
No Satellites
Temperature Compensated Crystal Oscillator (TCXO)
Better than 120 µs per hour
Rubidium (Rb_2) disciplined oscillator
1 s 10 s 100 s 1000 s 10000 s 100000 s
3E-11 1E-11 3E-12 3E-12 2E-12 8E-13
Precision OCXO (XTAL_1) disciplined oscillator
1 s 10 s 100 s 1000 s 10000 s 100000 s
2E-12 3E-12 1E-11 1E-11 3E-12 1E-12
Standard OCXO (XTAL_2) disciplined oscillator
1 s 10 s 100 s 1000 s 10000 s 100000 s
1E-9 2E-10 3E-10 3E-10 3E-11 1E-12
-Frequency Accuracy
-Tracking At Least One
Satellite
Temperature Compensated (TCXO) disciplined
oscillator
1 s 10 s 100 s 1000 s 10000 s 100000 s
1.5E-9 5E-10 4E-10 3E-10 3E-11 1E-12
Rubidium oscillator (Rb_2)
5 x 10
-11
per month
High Stability OCXO (XTAL_1)
1 x 10
-10
per day after 30 days of operation
Standard OCXO (XTAL_2)
5 x 10
-9
per day after 30 days of operation
Frequency Stability
Tracking No Satellites
Temperature compensated (TCXO) oscillator
5 x 10
-9
per day after 30 days of operation

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SPECIFICATION DESCRIPTION OPTION
Static Operation Standard
Deviation Position Average
(measured in northern
hemisphere over sixteen
24 hours averaged
positions)
2.3 meters N/S 1.9 meters E/W 5.0 meters Vertical
Mobile Operation
Maximum Operational
Velocity
460 meters per second
Disciplined Oscillator
Control Resolution (Rb_2)
3 x 10
-14
SPECIFICATION DESCRIPTION OPTION
Power LED (green)
Fault LED (red)
Time Valid LED (green)
Tracking Satellites LED (green)
Monitoring/Status
Indicators
Reject Data LED (yellow)
Output Monitors 9 output monitor LEDs at each output socket for
present signal (and status available via TxD1/RxD1)
Sounder Selectable audible marker of GPS receiver 1 PPS,
Internal 1 PPS, or none
Antenna
Receiver
Initialization
Oscillator control
Oscillator frequency
Configuration parameters
Synthesizer phase-locked loop
RAM
Built In Test
FLASH/EPROM
Manual Controls Primary power switch
4 way keypad Data Entry Option
2 line LCD display
Instrument: -10 to +50°C
Operating Temperature
Range Antenna: -40 to +75°C
Humidity 95% non-condensing
Power Supply 115/230 VAC ±10% 45 - 65 Hz
48 VDC DC Supply Options
24 VDC
Power Consumption 10 W typically
Case 19” 1U rack mounting 12.01 inches deep
Weight 12 lbs typically

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INPUTS AND OUTPUTS REF DESCRIPTION OPTION
Telecom Frequency
Options
See
Below
E1 2.048 MHz
T1 1.544 MHz
Alarm J14
5-PIN din
180 DEG
Free contacts from status monitor
changeover relay and center contact can
be linked to + 5 V or 0 V internally
TxD1/RxD1 RS232 (OR RS422 user
selectable inside)
Port 1 J13
9 way D
socket User commands select and control output
types, request time data, frequency
controller data, GPS data, and instrument
output status
TxD2/RxD2 RS232 (OR RS422 user
selectable inside)
Port 2 J12
9 way D
socket Automatic 1 per second time and status
output OR output on receipt of user time
request
Carries 5 VDC at center pin for powered
antennas
GPS Ant J11
50R BNC
4 V output at 40 mA load
Option J10
50R BNC
One digital (DCLS 0 to 5 V from 50R) time
code same as selected modulated time
code or is 1
st
pulsed output
User selectable 1
st
pulse rate in place of
the time code
Selected time code or pulse level 0 to 5 V
from 50R source
1
st
Programmable Pulse
Output
-1 pulse per 2 ms to 1 pulse per day, with
1 ms to 9999 ms pulse duration active
high or active low
-1 pulse per day, active high 100 ms is
the default value
-Pulse time accuracy (on and off) is UTC
±150 ns
2
nd
Programmable Pulse
Output
PCB
ONLY
CON6
PIN 3
-1 pulse per 2 ms to 1 pulse per day, with
1 ms to 9999 ms pulse duration active
high or active low
-1 pulse per minute, active high 100 ms is
the default value
-Pulse time is 350 µs early
1 PPS J9
50R BNC
1 PPS, 0 to 5 V square wave from 50R
One user selectable modulated time code
IRIG B AFNOR NASA36 VELA 2137
Level 2.17 Vpp carrier high level from 10R
Maximum 1.7 Vpp into 50R (internally
adjustable)
Code J8
50R BNC
IRIG B is the default selection
8 KHz square wave (Telecom frame rate)8 KHz J7
75R BNC Square wave 0 to 5 V from 75R

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INPUTS AND OUTPUTS REF DESCRIPTION OPTION
Outputs are link selectable (internally)
with the following options:
Telecom (2.048 MHz or 1.544 MHz as
selected above)
1 MHz (internal jumper selection)
5 MHz (internal jumper selection)
10 MHz (internal jumper selection)
Output level 0 to 5 V from 75R
Telco (two) J6 & J5
75R BNC
Output level square wave 2.37 Vpp into
75R (G703 ¶ 6)
Outputs are from Bandpass filter module
2, specified at time of purchase, with the
following options:
Telecom (2.048 MHz or 1.544 MHz as
selected above)
1 MHz
5 MHz
10 MHz
Each output level 1 Vrms from 50R, 75R,
or 120R by internal jumper selection (50R
is the default)
Sine wave 3 Vpp from 75R meets G703 ¶
10
Transformer isolation (75R connector
grounded to case)
Frequency accuracy and holdover similar
to tables above
Meets primary reference clock
requirements G811
Harmonic distortion -30 dBc or better
Freq 2 (two) J4 & J3
75R BNC
5 MHz or 10 MHz phase noise, see
Appendix II
Outputs are from Bandpass filter module
2, specified at time of purchase, with the
following options:
Telecom (2.048 MHz or 1.544 MHz as
selected above)
1 MHz
5 MHz
10 MHz
Output level 1 Vrms from 50R
Frequency accuracy and holdover as per
tables above
Transformer isolation (50R isolated
connectors)
Harmonic distortion -30 dBc or better
Freq 1 (two) J2 & J1
50R BNC
5 MHz or 10 MHz phase noise, see
Appendix II

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1.4 Controls, Indicators, and Connectors
Figure 1 and Figure 2 show the GPS8 Plus front panel, rear panel, indicators,
connectors, and controls. The following two tables below describe these
indicators, connectors, and controls.
INDICATOR DESCRIPTION
Power Green LED indicates that the primary power is
available.
Fault Red LED indicates one of the following:
1. Phase lock loop control voltage within
10% of its limit.
2. Synthesizer lost lock.
3. One or more output failures.
4. Processor malfunction.
5. No satellite reception for 8 hours.
Time Valid -Green LED indicates the time and position data
are synchronized since turned on.
-Turns off after no satellite reception for 8 hours.
Tracking Satellites Green LED indicates that at least one satellite is
being tracked.
Reject Data Yellow LED indicates that the last time data
frame was rejected (error bypass active and
frequency in holdover).
Display 2 lines, each with 24 characters low noise back-
lit liquid crystal display.
Keypad 4 way keypad for programming and display
control.
Table 1 GPS8 Plus Front Panel

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CONNECTOR/CONTROL DESCRIPTION
Primary Power 3 pin IEC socket for 115 VAC power
Fuse Main HT fuse for primary power supply
Alarm 5 pin 180° latching DIN socket
Port 1 9 way D type socket for serial RS232 or RS422
data communications
Port 2 9 way D type socket for serial RS232 or RS422
data communications
GPS Ant BNC socket (grounded) 50R for antenna signal
and power
Option
Option Status
-BNC socket (grounded) 50R DC pulse output
-Green LED showing pulse output is active
1 PPS
1 PPS Status
-BNC socket (grounded) 50R 1 PPS output
referenced to UTC
-Green LED showing 1 PPS output is active
Analog Code (IRIG B,
AFNOR, NASA36, VELA,
and 2137)
Analog Code Status
-BNC socket (grounded) 50R (with optional
galvanic isolation)
-Green LED showing analog code output is active
8 KHz
8 KHz Status
-BNC socket (grounded) 75R
-Green LED showing 8 KHz output is active
Telco
Telco Status 2
-BNC socket (grounded) 75R
-Green LED showing Telco output 2 is active
Telco
Telco Status 1
-BNC socket (grounded) 75R
-Green LED showing Telco output 1 is active
Frequency 2
Frequency 2 Status 2
-BNC socket (grounded) 75R
-Green LED showing frequency 2 output 2 is
active
Frequency 2
Frequency 2 Status 1
-BNC socket (grounded) 75R
-Green LED showing frequency 2 output 1 is
active
Frequency 1
Frequency 1 Status 2
-BNC socket (isolated) 50R
-Green LED showing frequency 1 output 2 is
active
Frequency 1
Frequency 1 Status 1
-BNC socket (isolated) 50R
-Green LED showing frequency 1 output 1 is
active
Table 2 GPS8 Plus Rear Panel

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2 Installation
2.1 Scope of Section
Section 2 describes the procedure prior to and during the GPS8 Plus installation.
2.2 Unpacking and Inspection
Carefully remove the GPS8 Plus from the shipping carton. The following items
should be included in the shipment:
•1 GPS8 Plus
•1 power cord
•1 user guide
2.3 Installation and Testing
The standard GPS8 Plus is housed in a 19” 1U rack mounting case, which can
be installed in a rack using the front panel mounting slots.
The active high gain antenna is a sealed omni-directional device, which must be
mounted with the cable exit at the bottom. For optimum performance the unit
should be located outside on a roof, away from objects which may impair satellite
visibility or generate high frequency interference.
Before connecting power to the receiver check that the power supply is in
accordance with the specification and/or details on the rear panel and check that
the power cord has a secure earth connection. An initial self test should be
carried out before final installation or connection to other units. The power
indicator will be illuminated. After about nine seconds the unit will test all the
software controlled indicators in the following order:
1. Fault
2. Time Valid
3. Tracking Satellites
4. Reject Data
If all LEDs are turned on simultaneously, an internal hardware fault has been
detected by the processor. The diagnostic messages from TxD1 (SK2/J13)
during power on should be examined for a fault report.
After the indicators have been tested they will be set to reflect the current status.
At this point the unit can be switched off and finally installed or configured as
described in section 3.

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3 Operating Instructions
3.1 Scope of Section
Section 3 covers the operation, initialization, and configuration of the GPS8 Plus.
It is assumed that the unit has been installed in accordance with section 2.3 and
that power has been applied to the unit.
3.2 Operation
During normal operation the GPS8 Plus automatically receives, analyses, and
processes data from up to eight GPS satellites simultaneously. This data is used
to provide time, frequency, and position information. The information is made
available to the user through a range of output interfaces.
In applications where the unit is to be used for general test and measurement
purposes, it can be switched on and off as necessary. However, if the unit is to
be used as a time and/or frequency standard it should be running continuously to
allow long term software processing to take place, which is essential if the unit is
to achieve its optimum performance. With the internal ovened oscillator XTAL_1
option, this process takes a minimum of 24 hours. The unit carries out automatic
position averaging and then automatically shifts into the known position to obtain
the highest possible time and frequency precision. The position averaging
process also takes place during the first 24 hours after the unit is switched on.
As described in section 2.3 when the GPS8 Plus is switched on it will carry out a
self-test routine before entering the operational program, which sets the correct
initial conditions for the receiver system. Satellite tracking is automatic, provided
that at least one is visible and the GPS8 Plus already has a valid position in its
battery backed receiver. Now time and frequency control can start within
approximately 1 minute. If three or more satellites are visible, the 3-D position
information will also be processed. The GPS8 Plus internal frequency standard
will warm up in 5 minutes (OCXO) or 10 minutes (Rubidium). At the end of the
warm up period the frequency controller is reinitialized.
Reception of at least one satellite is confirmed by the green Tracking Satellites
indicator being illuminated. Subsequent synchronization of the time to UTC (up
to 15 minutes from satellite reception) is confirmed by the green Time Valid
indicator being illuminated. The voltage control of the internal oscillator
maintains synchronization of all outputs with GPS. If the control voltage of the
internal oscillator is within 10% of the end-of-range, the red Fault indicator will
illuminate. This indicator also illuminates if any of the 10 outputs that should be
present are not detected by the internal output signal detector.

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The final phase and frequency locking process may take several hours
depending on the oscillator type fitted and satellite reception. Because the GPS8
Plus uses a patented "Intelligent Phase Locked Loop" system that incorporates a
specially developed dynamic time constant, the control parameters are
continuously modified to achieve lock in the shortest possible time to optimize its
performance for the best time and frequency accuracy.
Note that after a position has been successfully navigated, the number of tracked
satellites drops down to 4 or less, the satellite receiver automatically selects 2-D
positioning mode, and continues to recover time. If the number of tracked
satellites then drops down to 3, 2, or 1, the positioning mode is suspended but
time is recovered. If no satellites are tracked, the GPS8 Plus runs in ‘holdover’
continuing to maintain all the outputs. After 8 hours of lost reception the red
Fault indicator is turned on.
If the unit has a current satellite ephemeris at switch on, the start up time (after
system initialization has been completed) is approximately:
•Initial acquisition : 1 – 15 seconds
•Satellite re-acquisition : 25 seconds
•Tracking: 20 seconds
•Navigating: 1 minute from start up
•Position averaging complete: 24 hours from start up
Synchronization of the receiver time will normally be completed within 1½
minutes after the unit is switched on. If the receiver does not have an almanac
already backed up in memory after it has been switched on, it will take typically 8
or 9 minutes to acquire the necessary parameters to enable precise time
acquisition, as indicated by the Time Valid indicator on the GPS8 Plus front panel
or by the assertion of the Time Valid status bit in the time message.
For fixed position operation, precise averaging of the position is carried out by
the GPS8 Plus over 24 hours. The reason for this averaging is depicted in the
illustration below. The illustration shows the raw time data before processing is
carried out by the GPS8 Plus is improved by a factor of 3 when the GPS8 Plus
has averaged its position. The vertical scale on the graphs is the 1 PPS
deviations in nanoseconds.

MANUAL P/N 900000007 REV 2.0
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-5 0 0
-2 5 0
0
250
500
0 4 8 12
FUR4.LOG
E la p s e d tim e in H o u rs
TIME DEVIATION IN ns
RAW 1PPS DATA FROM GPS RECEIVER - POSITION AVERAGED
-5 0 0
-2 5 0
0
250
500
0 4 8 12
FUR3.LOG
Elapsed Tim e in Hours
TIME DEVIATION IN ns
RAW 1PPS DATA FROM GPS RECEIVER - POSITION FINDING
Illustration 1 GPS8 Plus Averaging Position Result

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3.3 Serial Communications
The GPS8 Plus has two asynchronous serial ports that communicate with the
user. They are suitable for connection to other units such as printers, computers,
and terminals. They can be configured as RS232 or RS422 ports by turning
around a simple header inside the unit. The options fitted to this unit are
identified in the specification (section 1.3). This section describes the options in
more detail.
Each serial port is accessible via its own 9 way D socket, J13 and J12
respectively. An RS232 and RS422 selection header is mounted inside of the
unit just behind each of the 9 way D sockets on the main PCB. The header has
five straps traversing a standard 16 pin dual-in-line footprint at one end. If
inserted with the straps at the right-hand end viewed from the front of the main
PCB, the configuration of the associated connector is RS422 as defined in the
table below:
PIN GPS8 PLUS RS422
FUNCTION DIRECTION
1 Ground 0 V
2 Not used -
3 Tx data- Output
4 Rx data+ Input
5 Ground 0 V
6 Rx data- Input
7 Not used -
8 Tx data+ Output
9 Not used -
Table 3 RS422 Connections J13 and J12 (Factory Default)
If the header is inserted with the straps at the left-hand end viewed from the front
of the main PCB, the configuration of the associated connector is RS232 as
defined in the table below:
PIN GPS8 PLUS RS232
FUNCTION DIRECTION
1 Ground 0 V
2 Tx data Output
3 Rx data Input
4 Not used Input
5 Ground 0 V
6 Not used (RS422 Rx data-)
7 Not used -
8 Not used (RS422 Tx data+)
9 Not used -
Table 4 RS232 Connections J13 and J12 (Factory Default)
Note that the unused pins (6 and 8) have RS422 signal connections, if a 5-way
jumper is used to select RS232. This can be eliminated by using a 3-way jumper
for RS232 selection. The 3 jumpers are at the left-hand end of the header
viewed from the front of the main PCB for RS232 selection.

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3.4 TxD1/RxD1 (Rear Panel J13) Baud Rate and Character Format
TxD1 is designed for communication with the user because it gives the user
access to the unit status and GPS reception information. RxD1 receives
commands and data requests from the user.
SW1 (a red 8-way switch pack) located on the main PCB controls TxD1/RxD1
baud rate and character format at J13. Prior to the alteration of the switch,
disconnect the power connection to the GPS8 Plus and practice all normal safety
precautions when opening the unit to access the 8-way DIL switch SW1.
SWITCH 1 2 3 4 5 6 7 8
ON For baud rate selection see
the table below
Parity Odd
parity
8 data
bits
2 stop
bits
OFF No parity Even
parity
7 data
bits
1 stop bit
Table 5a TxD1/RxD1 Character Format
SW1-1 SW1-2 SW1-3 SW1-4 BAUD
OFF OFF OFF OFF 75
ON OFF OFF OFF 110
OFF ON OFF OFF 134.5
ON ON OFF OFF 150
OFF OFF ON OFF 300
ON OFF ON OFF 600
OFF ON ON OFF 1200
ON ON ON OFF 1800
OFF OFF OFF ON 2400
ON OFF OFF ON 4800
OFF ON OFF ON 4800
ON ON OFF ON 9600
OFF OFF ON ON 9600
ON OFF ON ON 19200
OFF ON ON ON 19200
ON ON ON ON 19200
Table 5b TxD1/RxD1 Baud Rate
The example below is of the SW1 setting for 4800 baud, 8 data, no parity, and
1 stop:
OFF
!
!!
!!
!!
!!
!!
! !
!!
!!
!!
!
ON !
!!
!!
!!
!!
!!
!
12345678
Example 1 SW1 Factory Default Setting

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3.5 TxD1/RxD1 (Rear Panel J13) User Commands and GPS8 Plus
Responses
The GPS8 Plus asynchronous serial port RxD1/TxD1 provides time, position, and
status data on request from the user. Furthermore, it can accept user
commands. For example, to set the output time to UTC or local, the user must
set the local time offset from the UTC, set the pulse length on-time or period, and
set the duration of an output pulse in 1 millisecond units.
The user can obtain information from the GPS8 Plus by sending single ASCII
character requests terminated by <CR><LF>. The GPS8 Plus response
message structure is based on the NMEA-0183 standard for interfacing Marine
Electronics Navigation Devices. The same structure is used for commands from
the user that sets the GPS8 Plus operating parameters. User characters are
sampled at 1 millisecond intervals and will be missed if more than 1 character is
received per millisecond. If there is no time delay between the characters sent to
the GPS8 Plus, the maximum baud rate for the user commands is 4800.
The requested record will be directed to the GPS8 Plus serial port TxD1 output.
All characters transmitted by the GPS8 Plus are consistent with ASCII character
or control codes. When hexadecimal numbers are transmitted to communicate
status bit values they are transmitted as ASCII characters 0 through 9 and A
through F (A through F characters are always upper case characters).
Reserved characters are used to indicate the beginning and end of records in the
data stream and to delimit data fields within a record. As an exception the time
request may be a single character and optionally without termination to minimize
the overhead of obtaining a time stamp response from the GPS8 Plus.
Example 2 ASCII Character Request

MANUAL P/N 900000007 REV 2.0
20
The list of reserved characters is given in the table below:
CHARACTER HEX VALUE USAGE
$ 24 Start of record identifier
<CR><LF> 0D 0A End of record identifier
, 2C Record field delimiter
* 2A Checksum field delimiter
Table 6 Reserved Characters
The notation ‘CK’ indicates the optional checksum value of the message that is
computed by an exclusive-OR of all bytes between the ‘$’ and ‘*’ characters. The
‘$’, ‘*’, and checksum are not included in the checksum computation. For
commands sent to the GPS8 Plus, the checksum may be omitted in which case
the associated ‘*’ must also be omitted. The GPS8 Plus will recognize the <CR>
as the end of the message and will not attempt to verify the missing checksum. If
the checksum is attached by the user, it will be checked by the GPS8 Plus and
the message will be rejected if an error is detected. Errors in the format of the
message will also cause a rejection.
The list of available single character requests is given in the table below:
REQUEST RESPONSE
a<CR><LF> Version number, time, position averager, and output status information
b x<CR><LF> Sounder control, where x = 0, 1, or 2
c<CR><LF> 1
st
pulse output (local/UTC) time or pulse period, polarity, and pulse length
d<CR><LF> Dilution of precision values and satellites used
e<CR><LF> Modulated time code, pulsed output, TxD1 format, and TxD2 format
f<CR><LF> Frequency and phase controller data
h<CR><LF> Health status of satellites
j<CR><LF> Leap second date and UTC-GPS seconds offset value
k<CR><LF> 2
nd
pulse output (local/UTC) time or pulse period, polarity, and pulse length
l<CR><LF> Location and signal strength of satellites (up to three records)
n<CR><LF> Version number of firmware and GPS engine
o<CR><LF> Run UTC/local time and local time hours offset
p<CR><LF> Position of GPS receiver antenna
S<CR><LF> Additional information (including magnetic deviation)
t Precision time request *
Z See software reset command 3.6.16
Table 7 Single Character Requests
* The precision time request is handled as a priority. The ‘t’ request does not
need to be followed by a <CR><LF>. If a <CR><LF> is appended, it will be
ignored.